1. Field of the Invention
The present invention generally relates to multi-piece golf balls and more particularly to golf balls having at least one component made of a composition comprising a polybutadiene rubber/ionomer resin blend. The golf ball includes an inner core (center) and at least one surrounding core layer. Furthermore, the golf ball includes a cover comprising an inner and outer cover layer. Preferably, the surface hardness of the outer core layer is greater than the material hardness of the inner cover layer.
2. Brief Review of the Related Art
Multi-piece solid golf balls having a core and multi-layered cover comprising inner and outer cover layers are generally known in the industry. In such balls, the inner core is made commonly of a natural or synthetic rubber, such as polybutadiene, styrene butadiene, polyisoprene, or highly neutralized acid copolymers. The inner cover layer (or intermediate layer) is often made of an ethylene-based ionomer resin that imparts hardness to the ball. These ionomer acid copolymers contain inter-chain ionic bonding and are generally made of an α-olefin such as ethylene and a vinyl comonomer having an acid group such as methacrylic, acrylic acid, or maleic acid. Metal ions such as sodium, lithium, zinc, and magnesium are used to neutralize the acid groups in the copolymer. Commercially available ethylene-based ionomer resins are available in various grades and identified based on the type of base resin, molecular weight, and type of metal ion, amount of acid, degree of neutralization, additives, and other properties. The outer cover layer, which is disposed about the inner cover layer, may be made from a variety of materials including ionomers, polyamides, polyesters, and thermoplastic and thermoset polyurethane and polyureas. In recent years, thin outer covers made of polyurethanes have become popular, because such covers tend to provide the ball with a softer feel. In general, players are better able to place a spin on such soft-covered balls and control their flight pattern.
Manufacturers of golf balls use different materials to impart specific properties and features to the ball. For example, the resiliency and rebounding performance of the golf ball is based primarily on the core. The core acts as an “engine” for the ball. In general, the rebounding performance of the ball is based on its initial velocity after being struck by the face of the golf club and its outgoing velocity after making impact with a hard surface. More particularly, the “coefficient of restitution” or “COR” of a golf ball refers to the ratio of a ball's rebound velocity to its initial incoming velocity when the ball is fired out of an air cannon into a rigid vertical plate. The COR for a golf ball is written as a decimal value between zero and one. A golf ball may have different COR values at different initial velocities. The United States Golf Association (USGA) sets limits on the initial velocity of the ball so one objective of golf ball manufacturers is to maximize COR under these conditions. Balls with a higher rebound velocity have a higher COR value. Such golf balls rebound faster, retain more total energy when struck with a club, and have longer flight distance.
Golf balls containing multi-layered cores and covers are generally known. For example, Higuchi et al., U.S. Pat. Nos. 7,086,969 and 6,634,961 disclose multi-piece solid golf balls containing a solid core consisting of a center core and outer core, an inner cover layer, and an outer cover layer. The inner core and outer core layer are molded from rubber compositions comprising polybutadiene, peroxide, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane, zinc oxide, antioxidant, zinc acrylate, and zinc salt of pentachlorothiophenol. The center core has a JIS-c hardness of 40 to 60; the surface of the outer core has a JIS-c hardness of 75 to 95; the inner cover layer has a Shore D hardness of 50 to 80; and the outer cover layer has a surface hardness of 35 to 60. The outer cover layer has a lower Shore D hardness than the inner cover layer.
One objective of the present invention is to develop compositions that can be used to make a highly resilient core for a golf ball. The ball should also have high durability and impact strength. More particularly, it would be desirable to have a highly resilient golf ball containing a dual-core and dual-cover. The present invention provides golf ball compositions having such properties as well as other advantageous characteristics, features, and benefits.